Perhaps this has been answered by some before and I missed it, but here goes. I play with 2 APDGT's, have one going on two years now and the other around 8 months. Both have had close to 7-8 string jobs each. They just do not feel like they have the pop they once had, kkind of feels like I'm hitting with a 2x4 right now, not much feel or power. So my question is, after time and so many string jobs, does a racquet typically lose feel and pop? After how long should one consider replacing with new sticks (I know it's a personally preference, but there has to be some sort of literature on this isn't there)? Thanks.